In a typical prior art oil well logging scenario, a string of well logging tools having multiple sensors for measuring characteristics of the earth formation along the wall of a borehole is lowered via a cable to the bottom of the borehole. Geophysical data is recorded by way of the sensors as the cable is wound in using a precision winch. The depth in the earth at which the sensors on the logging tools are positioned as data is logged is determined by measuring the logging speed and cable depth. Devices such as a depth wheel measurement instrument and an axial accelerometer may be utilized.
Typically, measurements taken along the length of a borehole by logging tools are intended to provide indications of oil-bearing or gas-bearing strata in the earth. In the prior art, measurements of various characteristics or parameters of earth formations are usually obtained by combining measurements (data logs) taken by way of multiple sensors during a single pass through the borehole, or taken during different passes through the same borehole. When combining such measurements it is necessary that they be accurately correlated in depth with one another to be useful.
One problem is that the movement downhole of a tool with its sensors is usually not uniform. The non-uniform motion may be caused by such thing as: (a) damped longitudinal oscillations of the logging tool on the cable, (b) sticking and slipping of the logging tool against the sides of a borehole or the wall of casing in the borehole, and (c) irregular motion of the sensors that are mounted on mechanical arms that have independent motion with respect to the logging tool itself. As a result of non-uniform motion of the logging tool, data collected by a particular sensor on the logging tool at any specific depth in the borehole may not show as being recorded at the specific depth but at a different depth. Similarly, data from different sensors of the same tool may not show as having been recorded at the same depth for all those sensors.
To overcome the depth measurement problem in the prior art, a well logging tool for measuring depth in a borehole is disclosed in U.S. Pat. No. 5,019,978, issued May 28, 1991 to Allen Q. Howard, Jr. and David J. Rossi. The invention in that patent provides for estimation of a dominant mechanical resonant frequency parameter and of a damping constant parameter. These two parameters are taken into consideration when correcting an approximate indication of depth of a well logging tool to determine the actual, true depth of the well logging tool in a borehole.
Besides measuring depth in a borehole, well logging seeks to measure other physical characteristics along the sides of the borehole such as fractures, bed boundaries and bed dips. A major advance in borehole logging has been the development by Schlumberger of the Formation Microscanner (“FMS”), a borehole imaging system. The FMS processing technique is described in U.S. Pat. No. 4,468,623 issued Aug. 28, 1984 to Stanley C. Gianzero, David E. Palaith, and David S. K. Chan; and in U.S. Pat. No. 4,567,759 issued Feb. 5, 1986 to Michael P. Ekstrom and David S. K. Chan. The FMS system uses a tool having wall-engaging pads each carrying an array of electrical sensors distributed in the circumferential direction with respect to the axis of the borehole. Signal voltages generated by the sensors are sampled as the well logging tool moves along the borehole. The signals are processed and rendered visible, by photographic or other printout, or by cathode-ray tube display as a two-dimensional visible image is formed over logged segments of the borehole walls. In such images, bed boundaries can be visually identified from sharp visible contrasts in the images, which reflect sharp changes in resistivity at boundaries of the beds. The images thus obtained may exhibit a resolution on the order of 0.5 cm, allowing very fine details of the formation to be distinguished due to the number of sensors in the circumferential direction, and the high rate of sampling in the longitudinal direction.
U.S. Pat. No. 4,251,773, issued Feb. 17, 1981 to Michel Cailliau and Philippe Vincent teaches the use of signals from sensors to determine dip (inclination) and azimuth (strike) of bed boundaries. More specifically, this patent teaches a logging tool that has four substantially identical pads with sensors angularly distributed about the axis of the logging tool in a side-by-side relationship and adapted to engage the borehole wall at ninety-degree intervals. The sensors provide resistivity measurements of the respective sectors of the borehole wall engaged by the pads. As the logging tool is moved along the borehole wall, the sensors continuously provide signals measuring the resistivity of the adjacent earth formation. Sharp variations in resistivity indicate boundaries between different beds in the earth formation. The signals produced by the sensors at different angular positions of the pads are processed to provide information about the dip of bed boundaries, i.e., the orientations of the bed boundaries with respect to a terrestrial reference, and the azimuth of the dip.
Another tool that is lowered into or withdrawn from a borehole and has multiple sensors that measure properties such as resistivity along segments of a borehole wall is taught in U.S. Pat. No. 5,960,371 issued Sep. 28, 1999 to Naoki Saito, Nicholas N. Bennett and Robert Burridge. This patent teaches use of the Hough transform to extract dip and azimuth of multiple fractures and beddings from any type of borehole image with respect to a terrestrial reference. The method is also robust enough to account for noise or gaps in the images. The method can separate dips and azimuths of fractures from those of formations. Thus, it can detect and characterize other geometric features (e.g., linear, circular, or ellipsoidal shapes, some of which may represent vugs in carbonate reservoirs) present in the images.
As previously pointed out, when combining data of well logs it is necessary that they be correlated in depth in order to be useful. One method for depth correlation of well log data is taught in U.S. Pat. No. 4,327,412 issued Apr. 27, 1982 to John P. Timmons. The method disclosed in this patent is relatively complex and determines the displacement between a plurality of well logs so they may be correlated and combined. The well logs are derived from multiple, spaced sensors passed one time through a single borehole, or from separate passes of the same sensors through the same borehole. First, a normalized correlation function between selected groups of samples of the sets of data is determined as a first assumption to have a predetermined displacement relationship with the groups of samples of the data. A step of determining the normalized correlation function is repeated for a number of overlapping groups of well log data samples to produce a number of overlapping correlation functions. At least some of these overlapping correlation functions are combined to produce an improved correlation function that is used to depth correlate the well log data to a common, accurate depth level.
Another method for correlating well logging data collected by multiple sensors is taught in U.S. Pat. No. 6,272,232 issued Aug. 7, 2001 to Jean-Pierre Delhomme and Jean F. Rivest. This patent teaches a method for constructing, from an initial image of the wall of a borehole, a new “crossing-component image” centered on the axis of the borehole. The new image is representative of variations in a physical parameter of the earth formation in both the longitudinal direction of the borehole (depth), and in the peripheral direction of the borehole wall (laterally). The new image includes only those components of the physical parameters that extend all the way across the initial borehole image from one side of the image to the other. The method also includes determining variations in one or more attributes relating to the new image as a function of depth. The variations provide information relating to morphology to indicate solid zones, bedded zones, or different types of heterogeneous zones.
When individual well logs are concurrently obtained from a first and a second set of sensors vertically spaced from each other in a well logging tool the well logs also need to be depth correlated. They need to be depth correlated so that the data for any given level in the well taken by the two sets of vertically spaced sensors are aligned in order to be useful. Another method for depth correlating well log data is taught in U.S. Pat. No. 4,320,469, issued Mar. 16, 1982 to William J. Frawley and Philip A. Mongelluzzo. This patent teaches doing this by using data correlograms obtained by applying a correlation function to a pair of digitized well logs to reduce the amount of data that must be processed. The remaining data is then processed in an efficient and accurate manner in order to arrive at results indicating exactly how much two logs must be shifted with respect to each other for an optimized depth correlation between them.
While methods for depth correlating data obtained using sensors spaced vertically from each other on a well logging tool, or obtained at separate times of the same borehole are known, some of these methods are very complex and there is a need for a better method to depth correlate data obtained from well logging tool sensors.